libctf: new test of enum lookups with the _next iterator

I had reports that this doesn't work.  This test shows it working (and
also shows how annoying it is to do symbol lookup by name with the
present API: we need a ctf_arc_lookup_symbol_name for users that don't
already have a symtab handy).

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* testsuite/libctf-lookup/enum-symbol.lk: New symbol-lookup test.
	* testsuite/libctf-lookup/enum-symbol-ctf.c: New CTF input.
	* testsuite/libctf-lookup/enum-symbol.c: New lookup test.